3D scanning is a very effective technology for producing millions of spatial measurement points of objects within minutes or seconds. Increasingly, terrestrial 3D scanners are also used for classical surveying tasks and projects. At the moment, however, said scanners still have a few deficiencies or disadvantages, such as for example a field workflow that is atypical for geodetic surveying, insufficient measurement accuracy or the need for subsequent data processing in the office to prepare the desired measurement results.
Established manufacturers of laser scanners are currently working on matching the workflow management of 3D scanners to the needs of classical surveying. A first step here is to reduce the size of the scanner apparatuses and to improve their robustness so as to allow their field use in rough environments such as for example on construction sites. Portability and a flexible and quick setup of the measurement devices are essential requirements in surveying applications. Examples of such scanner apparatuses are the TRIMBLE GX 3D, Faro Photon 120 or ScanStation C10 by Leica Geosystems, such as is described for example in DE 20 2006 005643 U1 or US 2009/147319.
For classical surveying tasks such as for example recording buildings or civil engineering structures, the measurement accuracy of such prior art scanners is frequently too low. In particular in the case of steel structures, high point accuracy is unavoidable since the structural elements used in this case usually require narrow measurement tolerances. Attempts have therefore been made to achieve higher measurement accuracy with each development generation of such scanner apparatuses, not least to be in a position to cover new markets and further surveying tasks.
Examples of recent scanner applications are:                monitoring existing buildings with respect to changes in shape, subsidence, tilting etc;        monitoring the construction progress on construction sites and monitoring the dimensional accuracy of the work that has been carried out;        support during setup and alignment of various components on construction sites, such as for example columns, pipelines, openings, windows, doors, ready-mixed concrete parts etc.;        inventories of existing buildings for map representation or 3D modeling, for example when planning reconstructions or extensions;        forensic/criminal-technical applications, for example for digitally reconstructing events at incidence scenes or accident sites;        classical surveying tasks such as land surveys, spatial planning, marking of buildings, checking zoning regulations etc.;        historical restoration.        
In terms of basic construction, such scanners are configured to acquire, using a distance measuring unit, usually an electro-optical and laser-based distance measuring unit, a distance from a measurement point. A direction-deflection unit, which is likewise present, is configured here such that the measurement beam from the distance measuring unit is deflected in at least two spatial directions, as a result of which a spatial measurement region can be captured. The deflection unit can be realized in the form of a moving mirror or alternatively also by other elements suitable for controlled angular deflection of optical radiation, such as for example rotatable prisms, movable light guides, deformable optical components etc. Measurement is usually carried out by determining distance and angles, that is to say in spherical coordinates, which can also be transformed into Cartesian coordinates for representation and further processing purposes. The distance measuring unit can be designed for example according to the principles of time-of-flight (TOF) measurement, phase measurement, wave form digitizer (WFD) measurement or interferometric measurement. For quick and accurate scanners, in particular the measurement time must be short while simultaneously achieving high measurement accuracy, for example a distance accuracy in the millimeter range or below for measurement times of individual points in the range of microseconds or milliseconds.
The distance measuring units used in scanners frequently have internal reference sections, such that calibration of the distance measurement, in particular of the distance offset, can be carried out thereby and thus a high degree of accuracy of the distance measurement is attainable. The slope error or scaling error in today's distance measuring units is often much less than 1 ppm, and recalibration is therefore rarely necessary. However, the distance offset can change over time, field calibrations therefor are widely known today. In older devices, the point accuracy of scanners, as the maximum 3D distance deviation and thus error of the measured point in space, was often limited by the measurement accuracy of the distance measuring unit. More accurate distance measurements required correspondingly longer measurement times, as a result of which an increase in distance measurement accuracies was accompanied by a reduction in the measurement or scanning speed, as surveyed spatial points per unit time. With advances in the field of electro-optical distance measurement, fast to ultrafast laser distance measuring units that still have high distance measurement accuracy are available today.
The critical link in the overall system with respect to point accuracy has thus shifted from distance measurement accuracy to the area of angle measurement accuracy. In order to be able to meet the increased accuracy requirements, the long-term stability of the opto-mechanical system is also of increasing importance.
Commercial scanners with high accuracy nowadays achieve a point accuracy in the range of 5 to 20 mm for measurement distances below 200 m. For distances of up to 50 m, the achievable point accuracy is 2 to 6 mm, and for measuring distances of less than 25 m, a point accuracy of approximately 1 to 4 mm is certainly achievable. With respect to the direction accuracy or angle measurement accuracy, a section of 5 mm at a distance of 50 m for example corresponds to an angle measurement accuracy of 20″ (angular seconds) or approximately 100 μrad, which in the prior art already represents a high angle measurement accuracy.
Scanners of the highest precision that are currently available on the market can certainly be categorized into a product class having an accuracy of between 8″ and 12″, however, such scanner apparatuses with a specified direction accuracy or angle measurement accuracy of better than 20″, and in particular of better than 12″, are confronted with new problems which in less precise scanners are of no or merely subordinate importance. A 3D scanner having the high specified angle measurement accuracy mentioned above requires careful handling, use and care in order to retain said accuracy. In particular in mobile, portable apparatuses for field use, influences from the environment such as for example direct solar irradiation, bumps, impacts etc. must be expected, which can result in mechanical misadjustment and an accompanying reduced measurement accuracy. Such high-precision scanners are therefore suitable for use in a laboratory, but not in the field.
Known scanners also do not allow the end user to check the accuracy of the instrument in the field without undue effort, as is the custom with classical measurement devices. Any accuracy guarantee can at best be provided by the manufacturer, but this requires time-intensive and cost-intensive short check intervals and recalibration in the factory. The axis error values (for example in the form of angles and distances (offsets)) that are ascertained by the manufacturer, at an institute which is correspondingly equipped therefor, or at a national testing laboratory are usually directly input and stored in the instrument software.
In order to also introduce scanning measurement systems having a polar measurement principle, such as a terrestrial laser scanner, increasingly into geodetic practice, approaches have also been developed to examine calibration and checking of the measurement accuracy for these devices.
However, the error determination is here generally based on a construction which is analogous to the tachymetric measurement principle. However, scanners have an entirely different construction, which is why such an approach does not apply. In particular the error influences of the laser axis can differ significantly from those of a classical target axis.
A known calibration method for example uses a set of planes arranged in space, which are scanned several times in different instrument setups. On the basis of the known algorithms of combining points of view, the calibration method determines the accuracy of the system or calibration parameters from the identical object planes by utilizing one or more adjustment calculations.
To register scanned point clouds, as a combination of measurement data from individual scan cycles in different scanner setups, a transformation (for example Helmert transformation) over identical points can be carried out. The identical points are derived from the data of the scanned target marks, wherein, however, commercial target marks or targets can often not be surveyed by a scanner with the accuracy that is required herefor. An accuracy determination in such a setup, in which subsequently the measurement is also carried out, would also be preferable to the frequent position changes of this method. The entire calibration process is relatively complex, since for a sufficiently accurate determination of the instrument errors, for example more than ten planes in at least three setups are required, which is impractical for a routine field use in geodesy.
Another known method uses a known highly accurate reference point field (tie points), which is fixed for example in a hall on the walls and on the ceiling in the form of a large number of reflective target marks. The coordinates of these reflective target marks are initially calibrated exactly using a highly precise coordinate measuring machine such as a theodolite, and subsequently these predetermined coordinates which are known exactly are correlated with the scanner data by a mathematical calibration model. Such a procedure is too complex for general field use and therefore not practicable. Another disadvantage is the additional falsification of the angle calibration on account of latency times in the synchronization between angle and distance measurement. The ascertained spatial coordinates of reference objects or reference points are falsified on account of the dynamic scanning process, in particular in scanners with a fast rotation axis of typically 100 Hz. A synchronization delay of for example 100 ns in a scanner that rotates at 100 Hz already produces an object offset of 13 angular seconds which is tangential to the scanning movement. This error is reflected in the above method as an index error in the determination of the calibration parameters.
Also known is the method of reversal measurement commonly used in electronic theodolites in geodesy, which takes place as follows: the sighting device is used to measure exactly centrally and accurately to the second, that is to say with an accuracy in the range of a few angular seconds or less, one or more target marks in two telescope faces. The axis-relevant system parameters can be determined or the system accuracy can be verified on the basis of the measurement angles associated with the exact measurements. The system is here used in a direction of (X, Y) gon in the initial state and at approximately (X+200, 400−Y) gon in a rotating state, wherein all elements that contain errors and determine direction, such as the vertical axis, the trunnion axis and in particular the target axis of the sighting device, are rotated.
According to this principle, it is possible to determine a large number of errors, which in theodolites or analogously constructed instruments can substantially be represented by the following calibration parameters:
l,q: index error of the 2-axis tilt sensor, or of the vertical axis tilt
i: index error (angular offset) of the vertical angle sensor
c: collimation error of the collimation line
k: trunnion axis tilt.
A model having such parameterization or analogous parameterization will also be referred to as theodolite model below.
By way of example, the index and collimation errors are determined using a two-face measurement with a preferably horizontal collimation line, since with this arrangement the two types of error can largely be separated from the influences of the other parameters.
Although the laser scanner also makes it possible in principle to scan an object as described above in two alignments of the sensor head, the influences of the angle errors and the distances (offsets) of the scanner axes using point clouds from the first and second alignments cannot be ascertained and eliminated in the same form. For one, this is because exact point association is not always provided on account of the rastered surface-type measurement, especially with point densities in the raster of 5 mm and coarser. Another reason is because the calibration models have been entirely taken up by the theodolite or tachymeter model, and the latter do not correctly describe the measurement system of a scanner. As a result, non-real or even impermissible instrument parameters are determined and, in addition, their influences on the coordinate deviations are incorrectly modeled, which per se cannot lead to the intended improvements.
It is known in the prior art for example to record, for the reversal measurement in scanners, at least six white marking spheres made of wood as test objects in one scanning process in two alignments. The measurement values are here the coordinates of the captured portions of the sphere surfaces, from which the coordinates of the sphere center points are calculated and subsequently the axis errors are ascertained by modeling and adjustment calculation. The determination of the coordinates of sphere center points using scanners, however, largely contains gross errors. For example, at best systematic errors of approximately 3 mm can be achieved using the known three-dimensional test objects at a distance of 50 m, wherein the standard deviation is approximately of the same order of magnitude. Precise calibration is therefore not possible, in particular in conjunction with a device model as a basis which does not correctly describe the axis system of the scanner. The above-mentioned accuracies are simply not sufficient to calibrate errors of the axis system of a scanner.
For highly precise error determination, even in the above-mentioned reference point field that is known with a high level of accuracy, the residuals or the noise components are too great for an adjustment calculation of the scanner data with the reference data. In laser scanners, the above-mentioned classical parameter set of the reversal measurement is also no longer applicable in this form, and there is also no sighting device for a measurement that is accurate to the second.
Although the laser beam can at first glance be considered representative of the collimation line, it has entirely different properties with respect to the directional invariants. This new type of dependence or parameterization of the collimation line has up until now not been taken into consideration in this form in surveying, or it has either not been known at all up until now or at least its influence on the calibration was not known in this form.
As already mentioned, a corresponding robustness of the entire 3D scanner apparatus is also necessary to ensure the required angle measurement accuracies and to obtain highly accurate and reproducible measurement results. For this reason, the components for laser-beam guidance, in particular static or rotating deflection units, must have a high stiffness in order to be resistant to environmental influences. On the other hand, in addition to the aforementioned quick distance measurement, generic scanner apparatuses must also move the measurement laser beam with a correspondingly high speed so as to scan objects within a short period of time. This scan movement can be realized for example with quickly oscillating or rotating mirrors, for example at a high rotational speed or frequency of more than 200 rotations per second. For reasons of dynamics, it is necessary even at medium speeds, below the deflection frequency of 200 Hz mentioned by way of example, for the dimensions of the moving part to be kept as small as possible, which, however, is inconsistent with the requirement of high stiffness.
Users of classical surveying instruments such as theodolites or tachymeters are accustomed to being able to check the accuracy of their measurement system any time and without undue effort, be it for example in a space with known reference marks or in the field by measuring suitable targets in two faces. The known scanners do not provide this possibility, and the user will practically have no option for accurately determining or verifying the measurement accuracy of the device or of calibrating the device.